A conventional orthogonal excitation gas laser device includes a housing having a hermetically sealed structure in which a laser medium gas, such as CO2 gas, is enclosed. The housing is provided with discharging electrodes for discharge exciting the laser medium gas, a heat exchanger for cooling the laser medium gas, and a blower for circulating the laser medium gas. In the continuous oscillation of the gas laser, such as CO2 gas laser, discharge excitation is commonly used for excitation caused by collision of electrons during discharge in order to excite (pump) gas molecules which can emit laser light up to an energy level required for stimulated emission. In this case for ensuring stable discharge, the internal space of the oscillator hosing is required to keep a vacuum state of 30 to 60 Torr. Therefore, the oscillator hosing of the laser oscillator is required to have a hermetic sealing performance capable of keeping the vacuum state.
Further, since regular maintenance is required for the discharging electrodes, the heat exchanger, the blower, mirror optics and the like located within the housing, the heat exchanger, the blower and mirror optics located within the housing, it is desired that maintenance operators can easily access them from the external of the housing. Thus, the housing is commonly provided with a wide opening and a detachable cover member is attached thereto to occlude this opening.
Accordingly, it is desired that the cover member of the gas laser device can withstand a load caused when the interior of the housing is kept in a vacuum state, and can hold a hermetic sealing performance capable of keeping the vacuum state, and can have a wide opening area for easy maintenance of components inside the housing.
For such a cover member of the gas laser device, prior arts propose a plate-like one or a thin-walled structure having a cover member on which a curved surface expanding internally (See, e.g., Patent document 1) or externally (See, e.g., Patent documents 2 and 5) with respect to the housing is formed.
Further, gas laser devices, which are mainly used for laser machining, have been developed to produce higher power for enhancing the machining performance. Thus, longer discharging electrodes, larger heat exchangers and larger gas ducts are introduced, thereby resulting in a lengthened housing especially along the optical axis and enlarged height and width of the housing. Moreover, as the surface area of the enlarged housing is increased, a high atmospheric pressure is applied to the housing and the cover member, hence, which are required to have a higher strength and rigidity.